Boil the potatoes in deep lightly salted water for 15-20 minutes until tender enough to mash.
Meanwhile roughly chop and wash the cabbage leaves and cook in a saucepan with a little boiling water. Drain thoroughly and when cool enough to handly wring the greens lightly in your fist.
When the potatoes are ready drain thoroughly to avoid a wet mash, then add the 40g butter. Mash together well, then fold in the drained cabbage and herbs. Season with black pepper.
Roll into 8 patties, using a little flour on your hands if needed.
Heat a thin layer of olive oil and butter in a wide frying pan over a moderate heat. Add the patties and fry on one side for 5-6 minutes until pale gold and lightly crisp on the underside. Turn over and cook the other side in the same way.
Divide the watercress between two plates then add the patties. Add a fried egg or two if wished.